Refactor Package structure

This commit cleans up the package structure, to make two new top-level
packages `internal` (for internal-use packages) and `pkg` (for general
shared utility code).

pkg/unpack/resource.go

@@ -0,0 +1,131 @@
+package unpack
+
+import (
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+)
+
+var errExpectedFileButGotDirectory = errors.New("expected a file, but got a directory")
+var errExpectedDirectoryButGotFile = errors.New("expected a directory, but got a file")
+
+// InstallDir installs the directory at src within fsys to dst.
+//
+// onInstallFile is called for each file or directory being installed.
+//
+// If the destination path does not exist, it is created using [os.MakeDirs]
+// The directory is installed recursively.
+func InstallDir(dst string, src string, fsys fs.FS, onInstallFile func(dst, src string)) error {
+	// open the source file
+	srcFile, err := fsys.Open(src)
+	if err != nil {
+		return err
+	}
+
+	// stat it!
+	srcInfo, err := srcFile.Stat()
+	if err != nil {
+		return err
+	}
+
+	// make sure it's a file!
+	if !srcInfo.IsDir() {
+		return errExpectedDirectoryButGotFile
+	}
+
+	// call the hook (if any)
+	if onInstallFile != nil {
+		onInstallFile(dst, src)
+	}
+
+	// do the installation of the directory.
+	// the type cast should be safe.
+	return installDir(dst, srcInfo, srcFile.(fs.ReadDirFile), src, fsys, onInstallFile)
+}
+
+// installResource installs the resource at src within fsys to dst.
+//
+// OnInstallFile is called for each source and destination file.
+// OnInstallFile may be nil.
+func installResource(dst string, src string, fsys fs.FS, onInstallFile func(dst, src string)) error {
+	// open the srcFile
+	srcFile, err := fsys.Open(src)
+	if err != nil {
+		return err
+	}
+	defer srcFile.Close()
+
+	// stat it!
+	srcInfo, err := srcFile.Stat()
+	if err != nil {
+		return err
+	}
+
+	// call the hook (if any)
+	if onInstallFile != nil {
+		onInstallFile(dst, src)
+	}
+
+	// this is a directory, so the cast is safe!
+	if srcInfo.IsDir() {
+		return installDir(dst, srcInfo, srcFile.(fs.ReadDirFile), src, fsys, onInstallFile)
+	}
+
+	// this is a regular file!
+	return installFile(dst, srcInfo, srcFile)
+}
+
+func installDir(dst string, srcInfo fs.FileInfo, srcFile fs.ReadDirFile, src string, fsys fs.FS, onInstallFile func(dst, src string)) error {
+	// create the destination
+	dstStat, dstErr := os.Stat(dst)
+	switch {
+	case os.IsNotExist(dstErr):
+		if err := os.MkdirAll(dst, srcInfo.Mode()); err != nil {
+			return errors.Wrapf(err, "Error creating destination directory %s", dst)
+		}
+	case dstErr != nil:
+		return errors.Wrapf(dstErr, "Error calling stat on destination %s", dst)
+	case !dstStat.IsDir():
+		return errors.Wrapf(errExpectedDirectoryButGotFile, "Error opening destination %s", dst)
+	}
+
+	// NOTE(twiesing): We don't use fs.Walk here.
+	// If we did, we'd have to reconstruct relative paths.
+	// That would be very ugly!
+
+	// read the directory
+	entries, err := srcFile.ReadDir(-1)
+	if err != nil {
+		return errors.Wrapf(err, "Error reading source directory %s", srcFile)
+	}
+
+	// iterate over all the children
+	for _, entry := range entries {
+		if err := installResource(
+			filepath.Join(dst, entry.Name()),
+			filepath.Join(src, entry.Name()),
+			fsys,
+			onInstallFile,
+		); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func installFile(dst string, srcInfo fs.FileInfo, src fs.File) error {
+	// create the file using the right mode!
+	file, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, srcInfo.Mode())
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	// copy over the content!
+	_, err = io.Copy(file, src)
+	return errors.Wrapf(err, "Error writing to destination %s", dst)
+}